The present invention relates to a carrier, a classifier for classifying the carrier, a method of classifying the carrier, a method of preparing the carrier, a developer using the carrier and a process cartridge using the developer.
Electrophotographic developing methods include a one-component developing method using only a toner, and, a two-component developing method using a two-component developer including a carrier and a toner.
The carrier in the two-component developer expands a charged area of the toner, and therefore the two-component developer has more stable charge ability than the one-component developer. In this way, the two-component developer is more advantageous to produce quality images for long periods. Further, since the two-component developer has a high toner supply capacity to a developing area, it is widely used.
Recently, in order to improve image resolution, reproducibility, and colored images, a developing system capable of consistently developing a latent image is essential. Therefore, various methods are employed in terms of both process and developer (toner and carrier). In terms of the process, a closer developing gap, a thinner film of the photoreceptor and a smaller diameter of the writing beam are effectively used. However, the high costs and low reliability of these method are still to be improved.
A toner having a small particle diameter largely improves reproducibility of dot images. However, a developer including such a toner still poses difficulties, such as background fouling and insufficient image density.
On the other hand, a carrier having a small particle diameter is known to have the following advantages.
(1) Since the carriers have a large surface area together, an individual toner can sufficiently be charged and there is less low-charged or reverse-charged toner. In addition, the resultant images have good “dot” reproducibility with less scattered spots and blurred images around a reproduced dot.
(2) Since the carriers have a large surface area together, an average charge amount of the toner can be lower. Therefore, the carrier having a small particle diameter can offset disadvantages of the toner having a small particle diameter, and at the same time, can bring out advantages thereof.
(3) The carriers having a small particle diameter form a microscopic magnetic brush and the resultant image seldom has a tip imprint.
However, conventional carries having a small particle diameter tend to adhere to photoreceptors and fixing rollers, and have problems in practical application. The carriers which are likely to adhere thereto are carriers having smaller particle diameters, and therefore various classification methods of narrowing the particle diameter distribution have been suggested.
Among the various classification methods, a classification method using a sieve can improve classification, as compared to a classification method using a centrifugal force or an air blow, and can collect particles having a desired particle diameter at a high yield.
However, the classification method using a sieve is known to have a difficulty in making the particle diameter distribution of particles having a small mass narrow.
As a method of solving this problem, Japanese Laid-Open Patent Publication No. 2001-209215 discloses a method of efficiently cutting particles having a particle diameter less than 22 μm by imparting an ultrasonic vibration to a metallic mesh of a sieve to give an accelerated velocity to the particles in a direction of up and down to prepare a carrier having high durability and less adherence, wherein the carrier has a weight-average particle diameter (Dw) of from 25 to 45 μm, a content of the particles having a particle diameter not greater than 44 μm not less than 70% by weight, a content of the particles having a particle diameter not greater than 22 μm not greater than 7% by weight and a ratio (Dw/Dp) of the weight-average particle diameter to a number-average particle diameter (Dp) of from 1 to 1.30.
This method can efficiently pass particles having a small particle diameter through a mesh because an accelerated velocity is given to them in a direction of up and down to substantially move like particles having a large mass, i.e., a true specific gravity. Further, it is disclosed that an ultrasonic transducer with a resonant ring is used to improve efficiency of the sieve.
However, when a sieve has a mesh having small openings, since a mesh material is thin and a strength of the mesh is small (a thread is thin), an edge of the mesh is broken due to a weight of the carrier after used for a long time. Therefore, fine particles are mixed in the carrier having a desired particle diameter, resulting in a higher content of the fine particles.
When the mesh is clogged, the carrier hide among openings and it is quite difficult to remove the carrier, resulting in a need to replace the mesh.
Some meshes are woven with a resin thread, and usually with a stainless thread. Since the resin thread has a small stiffness, an ultrasound is not effectively transmitted to the mesh to classify.
On the other hand, production costs of a stainless mesh having small openings are extremely high, resulting in higher production costs of the carrier.
For at least the aforementioned reasons, a need exists for a method of preparing a carrier having a small particle diameter at low cost, which produces high quality images, and which has less adherence and a focused particle diameter distribution.